When making a synthetic natural gas from coal, the raw product gas must be upgraded to a high BTU pipeline quality gas. This is done by using the shift reaction to increase the hydrogen-carbon dioxide ratio in the raw gas. This mixture is then passed through a catalyst bed where it reacts exothermally to give a nominal yield of CH.sub.4 + H.sub.2 O.
A major problem in the design of methanator units is removal of heat from the catalyst. If the catalyst temperature is not held within relatively narrow limits the poisoning rate increases sharply and catalyst lifetime is reduced. An even more serious consequence of inadequate removal is the formation of hot spots, which can propagate and rapidly destroy the effectiveness of a catalyst.
One method heretofore employed for cooling methanator catalyst beds is to employ the heat capacity of the reacting mixture to remove the reaction heat. The gas is recycled through a methanator with heat removed in an external loop. Another approach in the prior art is to place the catalyst in pellet form in tubes that are surrounded by a heat transfer fluid. The reacting gas mixture flows through the tubes and by conduction and convection transfers the heat from the catalyst pellets to the tube wall and thence to the cooling liquid. In this approach an exceptionally large number of tubes is required for cooling since cooling is insufficient if the tube diameter exceeds about 12 mm.
A third cooling method employed in the prior art consists of vertical reentrant tubes, the exterior coated with a flame sprayed nickel catalyst. Heat generated by the reacting gas mixture flowing along an array of such tubes is removed by boiling heat exchange fluid. The heat exchange fluid is introduced into each reentrant tube by gravity flow through the inner tube of the concentric pair and flows up the annulus between the two tubes. Temperature is controlled by using an inert gas overpressure to control the boiling point. Since catalyst changes are required two or three times a year, each of the reentrant tubes would have to be removable so that the depleted catalyst could be taken off and replaced. This requires the use of many relatively large high pressure flanges which are difficult to put in the available space and are subject to leakage. A modification of this design to eliminate the necessity of the flanges is similar to the tube-in-vat approach described hereinabove except that the catalyst is applied by flame-spraying the inner wall of the tubes. In this approach catalyst removal and reapplication can be done in place. The tubes can be permanently welded to the end plates of the methanator. However, flame-spraying the catalyst on the inner wall of the tube requires an excessive amount of time since application rates are approximately 1 foot per hour and tube lengths are approximately 30 feet. It will be apparent that replacing the catalyst in a practical methanator containing as many as 1200 tubes is a time consuming operation.